Update README to reflect Managed VMs rebranding.
Update README to reflect Managed VMs rebranding.

Change-Id: Ic8074899ff5c545749d468f21e01bca6520c42e9

golang/appengine,fuchsia-mirror/third_party-github.com-golang-appengine,golang/appengine,fuchsia-mirror/third_party-github.com-golang-appengine